1. Field of the Invention
The present invention relates broadly to the treatment and recycle of rubber products and still more particularly to the use of recycled crumb rubber. The United States Government has rights to this invention pursuant to contract number DE-AC09-96-SR18500 between the U.S. Department of Energy and Westinghouse Savannah River Company.
2. Discussion of Background
Existing efforts to recycle used rubber, in particular used tires, into new rubber articles, especially tires, have met with only limited commercial success. In the United States alone, there are currently billions of tires stock-piled in long term storage with additional millions being added annually to such stock piles. Because of the large volume involved with tires, this discussion will be directed to tires although much of these comments are applicable to other new and used rubber products.
Because used rubber is usually processed in the form of crumb rubber, references herein will be to rubber in that form. A major limitation in the recycling of used tire material is that the used tire rubber can not be readily mixed in economical proportions to form new tire polymer mixes having acceptable properties.
During the vulcanization process of rubber, such as in the manufacture of new tires, accelerators, promoters, and/or initiators, are used and the formation of large numbers of sulfur crosslinks are established and are inherent in the cured rubber. It is generally believed in the art that the sulfur compounds which are present in used rubber, such as tire rubber, are deleterious in a subsequent curing process which uses used rubber as a component in a new polymer mixture. Formulations of tire rubber which use more than minor amounts of a used, previously vulcanized rubber result in a brittle cured end product unsuitable for many uses such as automobile or truck tires. Heretofore, efforts to reclaim scrap rubber have primarily included a physical sheering process which is suitable for a rubber which can be mixed with asphalt, forming asphalt rubber. Such use is taught in U.S. Pat. No. 5,304,576.
It is also known to take used rubber and depolymerize the vulcanized rubber in an organic solvent and then recover various polymerized fractions as taught in U.S. Pat. No. 5,438,078. Similarly, U.S. Pat. No. 5,264,640 teaches taking scrap rubber from used tires and regenerating the monomeric chemicals which are subsequently recovered. This method uses gaseous ozone to break down the crosslinked structure of the rubber followed by thermal depolymerization in a reaction chamber. U.S. Pat. No. 5,369,215 teaches a similar process in which used tire material may be depolymerized under elevated temperatures and at a reduced pressure to recover the monomeric compounds.
U.S. Pat. No. 5,891,926, which is incorporated herein by reference, is directed to a devulcanization process for used rubber in which elevated temperatures and pressures are used to partially devulcanize the rubber. Thereafter, a solvent 2-butanol is used to extract the devulcanized rubber from the non-rubber and/or solids component.
U.S. Pat. No. 4,104,205 teaches a method to devulcanize rubber from hose end trim and butyl tire bladders. While tire tread material was also treated, difficulties in exothermic reactions and physical properties of the microwaved materials were noted.
U.S. Pat. No. 4,341,667 teaches that the green strength of elastomers reclaimed through heat, microwave, chemical treatments, or physical shearing can be improved by the addition of butene polymers.
U.S. Pat. No. 4,469,817 teaches the microwave treatment of a vulcanized rubber. The treatment uses microwaves to raise the temperature of the rubber to a devulcanization temperature followed by rapid cooling with water.
U.S. Pat. No. 4,665,101 is an improvement to convectional heating of used rubber where microwave heating is additionally used. The combination of the heating methods is used to provide a more uniform heating profile and avoid extremes of localized temperatures within the treated material.
This application also relates to commonly assigned U.S. application having Ser. No. 09/542,392 having Attorney Docket No. WSR-14R, xe2x80x9cCombination Biological and Microwave Treatments of Used Rubber Productsxe2x80x9d, filed on Apr. 4, 2000 and which is incorporated herein by reference. Additionally, commonly assigned application Ser. No. 09/542,744 having Attorney Docket No. WSR-15R, entitled xe2x80x9cMicrobial Processing of Used Rubberxe2x80x9d, filed on Apr. 4, 2000, is incorporated herein by reference.
There remains a strong need for a practical, economical system for processing used tire parts such as tread rubber into a material which can be incorporated at a substantial loading level into new rubber compounding mixtures having good mechanical and chemical properties.
The present invention is a process and the resulting product of the process in which used rubber having relatively high surface reactivity has the reactive agents (as well as sulfur crosslinks) altered such that the resulting treated rubber may be incorporated into polymer mixes for high quality new rubber products, including tires, at much greater levels than used heretofore.
It is a further object of the present invention to provide a process and resulting product which permits the incorporation of large particle sizes of previously vulcanized rubber into polymer mixes for new rubber products.
In accordance with this invention, it has been demonstrated that particles of used crumb rubber can be treated with microwave energy which effectively alters the surface chemistry and reactivity of the so treated vulcanized crumb tire rubber. It has been found that the microwave treatment of the crumb rubber particle affects the surface reactivity of the crumb rubber particles to an extent that the rubber particles can be integrated into a new tire polymer mix at much higher levels than previously possible. The microwave treatment of the crumb rubber can be controlled so as to target only specific types of chemical bonds or agents by the selection of desired processing conditions, such as time and temperature, of the microwave bombardment. It has been found that the crumb rubber so treated is compatible with the new rubber polymer component of a tire mix.
The use of microwaves to target and modify select chemical species or bonds provides a process which can be carried out on an economical basis. The process is thought to be particularly useful in that the microwave treatment alters a wide variety of chemical additives used by tire manufacturers and which are present within the used rubber. The ability to target or alter these chemical constituents further enhances the usefulness and compatibility of the treated used rubber with virgin polymer.
In addition, the microwave treatment treats both the surface and interior bulk of the crumb rubber particle. While the surface treatment is believed necessary to bring about favorable co-compounding of used, previously vulcanized rubber with new tire rubber formulations, the microwave treatment is further believed to bring about favorable changes within the interior of the rubber particles. As used herein, the term xe2x80x9cbulk treatmentxe2x80x9d means the treatment of solid rubber particle with microwaves such that interior heating of the rubber particle occurs. The combination of heat and microwave radiation is believed to favorably alter the interior of the crumb rubber particles by the alteration of certain chemical constituents. As a result, a more uniform feed stock is provided and results in a crumb rubber product which is xe2x80x9csterilizedxe2x80x9d.
The use of the microwave treatment provides the potential of yielding a process and product which can be obtained on an economical basis at a commercial scale and in an environmentally acceptable manner.
It is thus an object of this invention to provide a microwave treatment process which alters by an effective amount the reactivity of used crumb rubber such that an increased amount of used crumb rubber can be mixed with a new tire polymer mix.
It is another object of this invention to provide a crumb rubber having an improved surface and bulk chemistry suitable for use in compounding a new tire rubber mix.
It is yet another object of this invention to provide a compounded rubber mix containing increased amounts of used crumb rubber.
It is yet another object of this invention to provide a treated rubber particle having a large size which may be incorporated into new rubber mix and provides a useful compounded final rubber product.
It is a further object of this invention to provide a process of devulcanizing a rubber particulate in which the devulcanization is substantially limited to targeted chemical bonds and which leaves certain desired physical properties of the particulate unaffected and or improved.
It is a further object of this invention to provide a process and material which increases the used rubber content of a molded rubber article while simultaneously improving the chemical and physical properties of the article.
These and other objects of this invention are provided by a process for devulcanizing and/or otherwise altering or modifying scrap rubber comprising: providing a supply of crumb rubber, said crumb rubber having a plurality of carbon-sulfur (Cxe2x80x94S) and sulfur-sulfur (Sxe2x80x94S) bonds; exposing the crumb rubber to microwave radiation having a frequency of 2450 MHZ; rendering an effective amount of the Cxe2x80x94S and Sxe2x80x94S bonds inactive; thereby, providing a crumb rubber having a treated surface defining an altered chemical state, the altered state being suitable for incorporation into a new rubber formulation and curing into a new rubber product.
A process for treating the crumb rubber is based upon the selection of a preferred frequency range of microwave energy which targets select chemical bonds. In particular, sulfur-carbon (Sxe2x80x94C) and sulfur-sulfur (Sxe2x80x94S) bonds of the crumb rubber are targeted by the microwave radiation at 2450 MHZ, though other microwave frequencies approved for use in countries other than the U.S. are also useful. Further, the raised temperature of the crumb rubber further facilitates the treatment of the crumb rubber. The microwave heating which results from the internal friction of polar molecules appears superior to externally supplied heat sources. The combination of heat and direct microwave irradiation is believed to destroy other constituents contained within the rubber particle, such as some accelerators, anti-oxidants, and stabilizers. Removal of these other constituents, particularly from the surface of the crumb rubber, improves the quality of the treated crumb rubber product for use in new rubber formulations.
In the initial vulcanization of tire rubber, sulfur and sulfur containing compounds, such as accelerators and initiators, are added which crosslink with the tire polymer. The bonding and crosslinkage stabilizes the polymer matrix, which imparts important desired properties to the rubber and increases the useful life of the tire. The presence of the sulfur compounds, and various additives present within used tire rubber, has limited the quantities of used tire rubber employed in new tire mixes. The sulfur containing compounds typically present on the rubber result in a final cured product which is often brittle and has other properties which make it unsuitable for many uses and especially for use as a tire.
As used in the present invention, the term xe2x80x9cdevulcanizedxe2x80x9d is used to indicate that certain surface and bulk properties of the particulate crumb rubber have been chemically altered by the application of microwave radiation. It is believed that the surface activity of mono, di, and polysulfides which formed polymer cross linkages during the initial vulcanization process have been reduced by the present microwave treatment process. As such, the crumb rubber is referred to here as xe2x80x9cdevulcanizedxe2x80x9d though it is understood that a substantial number of vulcanization products persist in the treated crumb rubber and in fact remain beneficial in the overall reformulations of new rubber mixes with the treated crumb rubber. Again, it is to be understood that other chemical and/or physical changes may take place which have a beneficial effect on the used rubber in reformulating it into new rubber products.
The work herein uses 40 mesh crumb truck tread rubber conforming to ASTM standard D5603. The crumb rubber was obtained from a commercial supplier of crumb rubber and is believed to represent a heterogenous mixture of different tire formulations from a variety of manufacturers. It is believed that the crumb rubber can be used over a wide range of particulate sizes, and the process is not size dependent upon the crumb rubber used. For example, standard sizes of 200-40 mesh, tire tread buffings, and even larger particles are believed to be useful.
The ASTM standard crumb rubber material used in the procedures set forth below was obtained from used truck treads. It is readily appreciated by those having ordinary skill in the art that crumb rubber originating from used passenger or truck tires will typically encompass products originating from many manufacturers and comprising a wide assortment of chemical constituents and compositions. Accordingly, a wide variety of different chemicals are expected to be present on the surface of untreated crumb rubber. This is particularly true for the reactive sulfur compounds originally added to the rubber during the vulcanization process. The present invention provides a treatment process which effectively modifies broad classes of reactive chemical constituents associated with the surface and bulk of crumb rubber. Accordingly, the treatment process will address expected variations encountered in a commercial source of crumb rubber.